Desalination membranes

ABSTRACT

Polymer-based membranes and methods for fabricating membranes are described. The methods include forming a casting solution featuring a polyvinylidene fluoride (PVDF)-based solvent and a polyvinylpyrrolidone (PVP)-based modifying agent, dispersing the casting solution to form a first element, generating a plurality of active sites on a surface of the first element, and forming a polymer-based membrane by exposing the surface of the first element to a fluorosilane composition to form a fluorosilane layer on the surface, where the fluorosilane composition includes a silane compound having at least one alkyl substituent that includes between 9 and 21 fluorine atoms.

TECHNICAL FIELD

The present disclosure generally relates to membranes for desalinationof water, and methods for fabricating such membranes.

BACKGROUND

Membrane desalination uses membranes, which are becoming a commoncommodity to extract freshwater from saline feedwater. The feedwater isforced onto the surface of a membrane, which selectively passes waterand retains salts.

SUMMARY

This specification describes super-hydrophobic, oleo-phobic, and antifouling polymer-based membranes for desalination of water, and methodsthat can be used to fabricate such membranes. The methods provide asimple and scalable chemical procedure to create membranes withsuper-hydrophobic, and the oleo-phobic characteristics at high contactangles. In general, the methods described herein for formation ofpolymer-based membranes involve phase inversion. Phase inversion is aprocess of controlled polymer transformation from a liquid phase tosolid phase. Different techniques can used to create phase inversionmembranes: precipitation from vapor phase, precipitation by controlledevaporation, thermally induced phase separation, and immersionprecipitation.

The methods include forming a casting solution using a polyvinylidenefluoride (PVDF)-based solvent and a polyvinylpyrrolidone (PVP)-basedmodifying agent. In some embodiments, the methods can also includeforming a casting solution using a polyvinylidene fluoride (PVDF)-basedsolvent, a polyvinylpyrrolidone (PVP)-based modifying agent, andmetal-based nanoparticles. The methods include dispersing the castingsolution to form a first element and forming a layer on top of the firstelement from a silane material with fluorinated alkyl substituents. Thefluorinated silane layer can be formed by methods such as dip-coatingand plasma etching. The first element and the fluorinated silane layerform the membrane. The fluorinated silane layer can includeheneicosafluorododecyltrichloro silane (CF₃(CF₂)₉CH₂CH₂SiCl₃), where thealkyl substituent on the silicon atom has twenty-one fluorine atoms(i.e., the alkyl substituent is designated an “F21” group).

Other fluorinated silanes can also be used in addition to, or as analternative to, heneicosafluorododecyltrichloro silane(CF₃(CF₂)₉CH₂CH₂SiCl₃). For example, more generally, the fluorinatedsilane layer can include one or more materials having the chemicalformula (CF₃(CF₂)_(n)(CH₂)_(m)SiCl₃). In general, n can be 3, 4, 5, 6,7, 8, 9, 10, 11, or 12, or even more than 12. Further, m can be 0, 1, 2,3, 4, 5, 6, or even more than 6.

In some embodiments, one or more of the —CH₂— units in the chemicalformula above can more generally be —CR₁R₂—, where each of R₁ and R₂ areindependently selected from the group consisting of: C₁-C₁₀ primary,secondary, or tertiary alkyl groups; C₂-C₁₀ alkene groups; C₂-C₁₀ alkynegroups; H; C₁-C₁₀ alkoxy groups; halogen groups (F, Cl, Br, I); hydroxylgroups; C₁-C₁₀ primary, secondary, or tertiary amine groups; C₂-C₁₀epoxide groups; C₂-C₁₀ ether groups; C₃-C₁₂ cyclic aromatic groups,which can optionally include from 1-3 heteroatoms (P, N, O, S); andC₂-C₁₀ cyclic alkyl groups, which can optionally include from 1-3heteroatoms (P, N, O, S).

In certain embodiments, one or more of the —Cl substituents of the Siatom in the chemical formula above can be a different substituent, sothat more generally, the silane species can be represented by theformula A-SiR′R″R′″. In this formula, A represents the substituent(CF₃(CF₂)_(n)(CH₂)_(m)— discussed above, while each of R′, R″, andR′″can be independently selected from the group consisting of: any ofthe types of groups (CF₃(CF₂)_(n)(CH₂)_(m)— discussed herein; C₁-C₁₀primary, secondary, or tertiary alkyl groups; C₂-C₁₀ alkene groups;C₂-C₁₀ alkyne groups; H; C₁-C₁₀ alkoxy groups; halogen groups (F, Cl,Br, I); hydroxyl groups; C₁-C₁₀ primary, secondary, or tertiary aminegroups; C₂-C₁₀ epoxide groups; C₂-C₁₀ ether groups; C₃-C₁₂ cyclicaromatic groups, which can optionally include from 1-3 heteroatoms (P,N, O, S); and C₂-C₁₀ cyclic alkyl groups, which can optionally includefrom 1-3 heteroatoms (P, N, O, S).

Without wishing to be bound by theory, it is believed that fluorinationof the silane species can impart super-hydrophobic, oleo-phobic, and/oranti-fouling properties to the membranes described herein. Accordingly,the silane species used typically have a high degree of fluorination,and carbon atoms present in substituents of Si may even beperfluorinated. More generally, for the silane species used to form themembranes described herein, the total number of non-C—C, non Si—Ccovalent bonding sites on C atoms can be j, where j=5 or more (e.g., 6,7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, or even more than30). The number of such sites occupied by F substituents, k, can be k=5or more (e.g., 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,21, 22, 23, 24, 25, 26, 27, 28, 29, 30, or even more). The degree offluorination, k/j, can be 0.50 or more (e.g., 0.55 or more, 0.60 ormore, 0.65 or more, 0.70 or more, 0.75 or more, 0.80 or more, 0.85 ormore, 0.90 or more, 0.95 or more, 0.98 or more, 1)

The methods described herein are simple, and scalable for thefabrication of super-hydrophobic and oleo-phobic membranes at a reducedtime using only a single step dip-coating approach. The methods alsoallow chemical modification of the membranes with or without plasmaetching, yielding membranes with super-hydrophobic properties and/oroleo-phobic properties, characterized by relative high water and oilcontact angles, respectively.

In this disclosure, a surface is “hydrophobic” if it exhibits a watercontact angle greater than 90°.

A surface is defined as “super-hydrophobic” if it exhibits a watercontact angle greater than or equal to 150°.

A surface is defined as “omniphobic” if it exhibits a water and oilcontact angles greater than 90°, where the oil used to measure the oilcontact angle is n-hexane.

A surface is defined as “oleo-phobic” if it exhibits an oil contactangle greater than 90°, where the oil used to measure the oil contactangle is n-hexane.

In this disclosure, the chemical group “Fa” refers to a primary,perfluorinated alkyl group with “a” fluorine atoms as substituents oncarbon atoms of the alkyl group. For example, the chemical group F9 hasthe structure CF₃(CF₂)₂CF₂—, the chemical group F17 has the structureCF₃(CF₂)₆CF₂—, and the chemical group F21 has the structureCF₃(CF₂)₈CF₂—. More generally, chemical group Fa has the structureCF₃(CF₂)_(p)CF₂—, where a=2p+5.

In some aspects, a method for fabricating a polymer-based membraneincludes: forming a casting solution including a polyvinylidene fluoride(PVDF)-based solvent, and a polyvinylpyrrolidone (PVP)-based modifyingagent; dispersing the casting solution to form a first element;generating a plurality of active sites on a surface of the firstelement; and forming a polymer-based membrane by exposing the surface ofthe first element to a fluorosilane composition to form a fluorosilanelayer on the surface; and the fluorosilane composition includes a silanecompound including at least one alkyl substituent comprising between 9and 21 fluorine atoms.

Embodiments of the method for fabricating a polymer-based membrane caninclude one or more of the following features.

In some embodiments, the method also includes exposing the surface ofthe first element to the fluorosilane composition by dip-coating thesurface in the fluorosilane composition to form the fluorosilane layer.

In some embodiments, generating the plurality of active sites includesplasma etching the surface of the first element.

In some embodiments, the method also includes forming the castingsolution includes dissolving a polyvinylidene fluoride-based agent in asolvent. In some cases, the polyvinylidene fluoride-based agent includesunsubstituted polyvinylidene fluoride. In some cases, the polyvinylidenefluoride-based agent includes at least one substituted polyvinylidenefluoride compound. In some cases, the polyvinylidene fluoride-basedagent includes unsubstituted polyvinylidene fluoride and at least onesubstituted polyvinylidene fluoride compound. In some cases, a molarpercentage concentration of the polyvinylidene fluoride-based agent inthe casting solution is between 5% and 20%.

In some embodiments, the method also includes forming the first elementby coagulating the dispersed casting solution in a coagulation bath.

In some embodiments, the method also includes dispersing the castingsolution on a surface using a casting knife.

In some embodiments, the fluorosilane composition includes at least onefluorinated chlorosilane compound. In some cases, the at least onefluorinated chlorosilane compound includes at least one alkylsubstituent covalently bonded to a silicon atom of the at least onechlorosilane compound, and the at least one alkyl substituent includesat least 9 fluorine atoms. In some cases, the fluorinated chlorosilanecompound includes3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heneicosafluorododecyltrichlorosilane(CF₃(CF₂)₉CH₂CH₂SiCl₃). In some cases, a weight percentage of the atleast one fluorinated chlorosilane compound in the membrane is between 5weight % and 50 weight %.

In some aspects, a polymer-based membrane includes: a first element thatincludes a polyvinylpyrrolidone (PVP) modifying agent, and apolyvinylidene fluoride (PVDF)-based compound; a fluorosilane layerdisposed on the first element and comprising at least one fluorinatedchlorosilane compound; a water contact angle for the membrane is greaterthan 150°; and an oil contact angle for the membrane is greater than100°.

Embodiments of the polymer-based membrane can include one or more of thefollowing features.

In some embodiments, the at least one alkyl substituent is a primaryalkyl substituent that comprises 9, 17, or 21 fluorine atoms.

In some embodiments, the at least one fluorinated chlorosilane compoundincludes a chlorosilane compound comprising at least two hydrocarbonsubstituents covalently bonded to a silicon atom of the chlorosilanecompound, and at least one of the at least two hydrocarbon substituentsincludes fluorine atoms.

Without wishing to be bound by theory, it is believed that the presenceof fluorinated alkyl substituents achieves chemical modification of themembrane with improved characteristics relative to conventionalmembranes used for desalination. For example, the improvedcharacteristics can include, but are not limited to, repulsion ofsurface contaminants and reduced fouling, leading to longer servicelifetimes and increased throughput during service. The presence ofmetal-based nanoparticles also enhances the mechanical strength of thepolymer-based membranes.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description. Other features and advantages will beapparent from the description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are schematic views of an example of a polymer-basedmembrane.

FIGS. 2A and 2B are flowcharts showing methods for fabricatingpolymer-based membranes.

FIG. 3 is a chart showing the wetting behavior of a plasma etchedpolymer-based membrane as a function of a concentration of a fluorinatedsilane constituent.

FIG. 4 is a chart showing the wetting behavior of plasma etchedpolymer-based membranes processed different dip-coating times to modifythe surface of the membrane.

FIG. 5 is a chart showing the comparative wetting behavior of variouspolymer-based membranes fabricated according to different methods.

FIG. 6 is a chart showing the wetting behavior of a polymer-basedmembrane with TiO₂ nanoparticles treated with different concentrationsof a fluorinated silane constituent.

FIG. 7 is a chart showing the comparative wetting behavior of variouspolymer-based membranes with TiO₂ nanoparticles fabricated according todifferent methods.

DETAILED DESCRIPTION

Membranes can be used for water desalination. Particularly, membranesmade from hydrophobic polymers such as polypropylene (PP),polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE) canbe used for water desalination. Membranes made from PVDF exhibitexcellent chemical, mechanical, and thermal properties with highporosity. However, organic pollutants such as grease, oil, and other lowsurface tension contaminants effectively wet the hydrophobic membranesand reduce their hydrophobic characteristics.

Methods such as tape casting, hot pressing, electro-spinning, and phaseinversion are commonly used in the fabrication of polymeric membranes.The phase inversion approach is a facile and scalable method for thefabrication of highly porous large-scale polymeric membranes on a smoothglass surface. This specification describes methods that can be used tofabricate super-hydrophobic, oleo-phobic, and anti-fouling polymer-basedmembranes for desalination of water and other applications.

FIGS. 1A and 1B are schematic views of an example of a polymer-basedmembrane 100, respectively. The polymer-based membrane 100 includes afirst element 102 that provides a base and support for the membrane. Themembrane 100 also includes a fluorosilane layer 104 disposed on top ofthe first element 102. The fluorosilane layer 104 improves thesuper-hydrophobic, oleo-phobic, and anti-fouling properties of themembrane 100. In general, elements 102 and 104 have differentcompositions, and as a result, their physical properties differ.

In some embodiments, element 102 includes a first component thatincludes polyvinylidene or a substituted polyvinylidene. For example, incertain embodiments, the first component includes polyvinylidenefluoride (CH₂CF₂)_(n). In some embodiments, the polyvinylidene fluoridecan have an average molecular mass of between 35,000 grams per mole(g/mol) and 65,000 g/mol (e.g., between 35.000 g/mol and 60,000 g/mol,between 40,000 g/mol and 60,000 g/mol, between 40,000 g/mol and 55,000g/mol, between 45,000 g/mol and 55,000 g/mol, about 53,400 g/mol). Ingeneral, the molecular mass of polyvinylidene fluoride is selected basedon the physical properties that are imparted to the membrane, andinteractions between polyvinylidene fluoride and other components of themembrane.

In certain embodiments, element 102 includes a first component thatincludes one or more substituted polyvinylidene fluoride polymers.Examples of such polymers include, but are not limited to,polyvinylidene fluoride polymers that include one or more C₁₋₁₀ linearor branched alkyl substituents, one or more C₂₋₁₀ linear or branchedalkenyl substituents, one or more C₂₋₁₀ linear or branched alkynylsubstituents, one or more halide substituents, one or more alkoxysubstituents, and one or more cyclic C₃₋₁₀ aliphatic substituents,optionally including from 1-3 heteroatoms selected from O, N, S, and P,one or more C₃₋₁₅ aromatic substituents, optionally including from 1-5heteroatoms selected from O, N, S, and P. Such polymers can includecombinations of any one or more of these substituents in every monomerunit, in every second monomer unit, in every third monomer unit, andmore generally, in every n^(th) monomer unit, where n is 2 or more (forexample, 3 or more, 4 or more, 5 or more, 6 or more, 8 or more, 10 ormore, or even more).

Examples of such polymers also include copolymers of different monomers,individually substituted with any of the substituents described herein.Suitable copolymers can include diblock copolymers, triblock copolymers,and copolymers of higher order (such as quaternary block copolymers).Further still, examples of such polymers include blends of any of theforegoing polymers of the first component. For example, blends of PVDFwith P(MMA-r-MnG), blends of PVDF with polyethyleneimine branched(polyion) components, blends of PVDF with a branched component havingpoly(acrylic acid) side chain, or a blend of a polysulfone base withanother miscible component.

In some embodiments, element 102 includes a second component thatincludes polyvinylpyrrolidone (C₆H₉NO)_(n). In certain embodiments, thepolyvinylpyrrolidone can have an average molecular mass of between 7,000g/mol and 13,000 g/mol (e.g., between 7,500 g/mol and 13,000 g/mol,between 7,500 g/mol and 12,500 g/mol, between 8,000 g/mol and 12,500g/mol, between 8,000 g/mol and 12,000 g/mol, between 8,500 g/mol and12,000 g/mol, between 8,500 g/mol and 11,500 g/mol, between 9,000 g/moland 11,500 g/mol, between 9,000 g/mol and 11,000 g/mol, about 10,000g/mol).

In some embodiments, the second component includes one or more polymersin addition to, or as alternatives to, polyvinylpyrrolidone. Examples ofsuch polymers include, but are not limited to, polytetrafluoroethylene(PTFE), polypropylene (PP), and polyethylene (PE).

In some embodiments, the second component includes one or moresubstituted polyvinylpyrrolidones. Examples of such polymers include,but are not limited to, polyvinylpyrrolidone polymers that include oneor more C₁₋₁₀ linear or branched alkyl substituents, one or more C₂₋₁₀linear or branched alkenyl substituents, one or more C₂₋₁₀ linear orbranched alkynyl substituents, one or more halide substituents, one ormore alkoxy substituents, and one or more cyclic C₃₋₁₀ aliphaticsubstituents, optionally including from 1-3 heteroatoms selected from O,N, S, and P, one or more C₃₋₁₅ aromatic substituents, optionallyincluding from 1-5 heteroatoms selected from O, N, S, and P. Suchpolymers can include combinations of any one or more of thesesubstituents in every monomer unit, in every second monomer unit, inevery third monomer unit, and more generally, in every n^(th) monomerunit, where n is 2 or more (for example, 3 or more, 4 or more, 5 ormore, 6 or more, 8 or more, 10 or more, or even more).

Examples of such polymers also include copolymers of different monomers,individually substituted with any of the substituents described herein.Suitable copolymers can include diblock copolymers, triblock copolymers,and copolymers of higher order (such as quaternary block copolymers).One example of a suitable copolymer is polyvinylidenefluoride-co-hexafluoropropylene (PVDF-HFP), but more generally,copolymers of any of the monomers described herein can be used. Furtherstill, examples of such polymers include blends of any of the foregoingpolymers of the first component. For example, blends of PVDF withP(MMA-r-MnG), blends of PVDF with polyethyleneimine branched (polyion)components, blends of PVDF with a branched component having poly(acrylicacid) side chain, or a blend of a polysulfone base with another misciblecomponent.

In some embodiments, a weight percentage of the first component of firstelement 102 described above is between 5% and 50% (e.g., between 5% and45%, between 10% and 45%, between 15% and 45%, between 15% and 40%,between 20% and 40%, between 20% and 35%, between 25% and 35%, between5% and 35%, between 5% and 30%, between 10% and 35%, between 10% and30%, or any other range within these ranges).

In certain embodiments, a molar percentage of the first component infirst element 102 is between 10 mole % and 50 mole % (e.g., between 10mole % and 45 mole %, between 15 mole % and 45 mole %, between 15 mole %and 40 mole %, between 20 mole % and 40 mole %, between 20 mole % and 35mole %, between 25 mole % and 35 mole %, between 10 mole % and 40 mole%, between 10 mole % and 30 mole %, between 15 mole % and 30 mole %, orany other range within these ranges).

In some embodiments, a weight percentage of the second component offirst element 102 described above is between 1% and 50% (e.g., between1% and 45%, between 5% and 45%, between 15% and 45%, between 15% and40%, between 20% and 40%, between 20% and 35%, between 25% and 35%,between 5% and 35%, between 5% and 30%, between 10% and 35%, between 10%and 30%, or any other range within these ranges).

In certain embodiments, a molar percentage of the second component infirst element 102 is between 10 mole % and 50 mole % (e.g., between 10mole % and 45 mole %, between 15 mole % and 45 mole %, between 15 mole %and 40 mole %, between 20 mole % and 40 mole %, between 20 mole % and 35mole %, between 25 mole % and 35 mole %, between 10 mole % and 40 mole%, between 10 mole % and 30 mole %, between 15 mole % and 30 mole %, orany other range within these ranges).

In some embodiments, first element 102 includes nanoparticles as shownin FIG. 1B. In general, nanoparticles function to adjust both chemicaland physical properties of first element 102. For example, in certainembodiments, nanoparticles can increase the mechanical strength (thecompressive strength, tensile strength, or resistance to shear) of firstelement 102. In some embodiments, nanoparticles can adjust chemicalproperties of first element 102. As an example, nanoparticles can beused to control functionalization, and more generally, the chemicalreactivity of first element 102.

A variety of different types of nanoparticles can be present in firstelement 102. In some embodiments, first element 102 can include one ormore different types of metal-based nanoparticles. Examples of suchnanoparticles include, but are not limited to, titanium dioxide (TiO₂)nanoparticles, silicon dioxide (SiO₂) nanoparticles, zinc oxide (ZnO)nanoparticles, iron oxide (Fe₂O₃) nanoparticles, copper oxide (CuO)nanoparticles, and aluminum oxide (Al₂O₃) nanoparticles.

First element 102 can also include multiple different types ofnanoparticles, including combinations of 2 or more (e.g., 3 or more, 4or more, 5 or more, or even more) different types of nanoparticles. Anyof the different types of nanoparticles discussed herein can be used insuch combinations.

In some embodiments, a weight percentage of nanoparticles in firstelement 102 can be between 1.0 weight % and 5.0 weight % (e.g., between1.0 weight % and 4.5 weight %, between 1.0 weight % and 4.0 weight %,between 1.0 weight % and 3.5 weight %, between 1.0 weight % and 3.0weight %, between 1.5 weight % and 5.0 weight %, between 1.5 weight %and 4.5 weight %, between 1.5 weight % and 4.0 weight %, between 1.5weight % and 3.5 weight %, between 2.0 weight % and 5.0 weight %,between 2.0 weight % and 4.5 weight %, between 2.0 weight % and 4.0weight %, or any range within one of the foregoing ranges).

In certain embodiments, an average numerical density of nanoparticles infirst element 102 is between 1.0 μm⁻³ and 25 μm⁻³ (e.g., between 2.0μm⁻³ and 25 μm⁻³, between 2.0 μm⁻³ and 20 μm⁻³, between 3.0 μm⁻³ and 20μm⁻³, between 3.0 μm⁻³ and 15 μm⁻³, between 4.0 μm⁻³ and 15 μm⁻³,between 5.0 μm⁻³ and 15 μm⁻³, between 5.0 μm⁻³ and 10 μm⁻³, or any rangewithin one of the foregoing ranges). Suitable nanoparticles for use infirst element 102 can be prepared using a variety of methods.

Examples of such methods are described in the following references, theentire contents of each of which are incorporated by reference herein:Yu et al., “A poly(arylene ether sulfone) hybrid membrane using titaniumdioxide nanoparticles as the filler: Preparation, characterization andgas separation study”, 29 (2016) 26-35; Kwak et al., “Hybridorganic/inorganic reverse osmosis (ro) membrane for bactericidal antifouling. 1. Preparation and characterization of TiO2 nanoparticleself-assembled aromatic polyamide thin-film-composite (TFC) membrane”,Environmental Science and Technology, 35 (2001) 2388-2394; Khorshidi etal., “Robust fabrication of thin film polyamide-TiO₂ nanocompositemembranes with enhanced thermal stability and anti-biofoulingpropensity”, Scientific Reports, 8 (2018) 1, DOI:10.1038/s41598-017-18724-w; Bagheripour et al., “Fabrication ofpolyvinyl chloride based nanocomposite nanofiltration membrane:investigation of SDS/Al2O3 nanoparticle concentration and solvent ratioeffects”, Asia-Pacific Journal of Chemical Engineering, 10 (2015)791-798. Certain nanoparticles for use in first element 102 areavailable commercially as follows: TiO₂- and Al₂O₃-based nanoparticlesare available from Sigma Aldrich, St. Louis, Mo., and SiO₂- andZnO-based nanoparticles are available from Cerion Nanomaterials,Rochester, N.Y.

In some embodiments, element 104 includes a first component thatincludes a silane compound with fluorine-containing substituents.Without wishing to be bound by theory, it is believed that the silanecompound can impart super-hydrophobic and oleo-phobic properties to themembrane, depending upon the nature of the silane compound, thecomposition of element 104, and of the overall membrane. It is alsobelieved that the silane compound can, depending upon the nature of thecompound, effectively adjust certain chemical properties of element 104.As an example, the silane compound can be used to controlfunctionalization, and more generally, the chemical reactivity ofelement 104.

A variety of different types of fluorinated silanes can be present inelement 104. In some embodiments, element 104 can include a silanecompound with one or more different types of fluorinated alkyl groups.Examples of such groups include, but are not limited to, silanes withone or more alkyl groups having nine fluorine atom substituents (e.g.,F9 groups), silanes with one or more alkyl groups having seventeenfluorine atom substituents (e.g., F17 groups), and silanes with one ormore alkyl groups having twenty one fluorine atom substituents (e.g.,F21 groups). In some embodiments, a single type of fluorinated silanecompound is used to form element 104. Alternatively, in certainembodiments 2 or more different types of silane compounds (e.g., 3 ormore, 4 or more, 5 or more, or even more) can be used to form element104. Further, in some embodiments, each silane compound contains only asingle type of fluorine-containing substituent, while in certainembodiments, one or more of the silanes used to form element 104includes 2 or more (e.g., 3 or more, 4 or more, 5 or more, 6 or more, oreven more) different types of fluorine-containing substituents. As anexample, element 104 can include a silane compound with one or more F9and one or more F17 groups, a silane compound with one or more F9 andone or more F21 groups, a silane compound with one or more F17 and oneor more F21 groups, and/or a silane compound with one or more F9 groups,one or more, F17 groups, and one or more F21 groups. More generally,combinations of any 2 or more of the silane compounds described hereincan be present in element 104, and each of the silane compounds caninclude one or more, two or more, three or more, four or more, five ormore, six or more, or even more of the different types offluorine-containing substituents described herein.

In some embodiments, a weight percentage of all fluorinated silanecompounds in element 104 can be between 5 weight % and 50 weight %(e.g., between 10 weight % and 45 weight %, between 10 weight % and 40weight %, between 10 weight % and 35 weight %, between 10 weight % and30 weight %, between 15 weight % and 50 weight %, between 15 weight %and 45 weight %, between 15 weight % and 40 weight %, between 15 weight% and 35 weight %, between 20 weight % and 50 weight %, between 20weight % and 45 weight %, between 20 weight % and 40 weight %, or anyrange within one of the foregoing ranges). In particular, the inventorshave discovered that a weight percentage between 5 weight % and 25weight % of the fluorinated silane compounds can impartsuper-hydrophobicity to the membrane. Super-hydrophobicity was observed,for example, when the fluorinated silane compound present in element 104was F21-CH₂CH₂SiCl₃, which has the chemical name3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-Heneicosafluorododecyltrichlorosilane.

FIG. 2A is a flowchart showing a method 124 for fabricating apolymer-based membrane 100. At step (126) the method includes forming acasting solution that includes a PVDF-based solvent, and a PVP-basedmodifying agent. In some examples, the casting of the PVDF solutionincludes one or more of tape casting, hot pressing, or electro-spinning.In an example of this method, a 15% PVDF-based solvent in N, N-dimethylformamide (DMF) solvent (e.g., N, N-dimethylformamide (C₃H₇NO; 99%)) wasmagnetically stirred for 24 hours at a temperature of 40 Celsius (° C.)in a glass vessel. Next, a 5% solution of PVP was added followed by thesame stirring period of 24 hours and at a temperature of 40° C. The PVPserves as a good adhesive, and a pore-forming agent.

In general, the percentage of the PVDF-based solvent in the castingsolution can be adjusted to control the properties of first element. Inthe example above, the PVDF-based solvent was present at a concentrationof 15 mole % in the DMF solvent. More generally, the PVDF-based solventcan be present at a concentration of between 5 mole % and 20 mole %(e.g., between 7 mole % and 20 mole %, between 10 mole % and 20 mole %,between 7 mole % and 17 mole %, between 10 mole % and 17 mole %, or anyrange within these ranges) in the DMF solvent.

At step (128) the casting solution is dispersed to form a first element102. For example, the homogeneous and bubble-free PVDF-based solutionwas cast on a glass surface with the help of a casting knife and kept atroom temperature for 5 minutes. Next, the casted solution was placed ina first coagulation bath (e.g., consisting of pure ethanol (e.g., C₂H₆O,99%)) for 5 minutes, and then it was placed in a second coagulation bath(e.g., consisting of 5-25% n-propanol (e.g., C₃H₈O) in distilled water)for 24 hours at ambient conditions. The first element 102 was removedfrom the second coagulation bath and rinsed several times with deionizedwater and dried at room temperature for 24 hours. The coagulation bathfacilitates the precipitation of casted polymer-based membrane to form asolid membrane matrix.

At step (130) a plurality of active sites is generated on a surface ofthe first element 102. In an example, the active sites were generated byplacing the first element 102 or the PVDF-based membrane into a plasmacleaner (e.g., Harrick Plasma Cleaner (PDC-32 G)) for etching. Plasmaetching was performed at a pressure of 100 millibars (mbars) for 10minutes to generate active sites on the surface of the first element102.

In general in step (130), a variety of different methods can be used togenerate active sites on the surface of element 102. For example, incertain embodiments, such methods can include wet etching, ionsputtering, sand blasting, reactive-ion etching, and combinationsthereof. In some embodiments, the process can generate a density of theactive sites as a percentage of the total surface area on the surface ofmembrane 100 that can be between 25% and 90% (e.g., between 30% and 90%,between 30% and 85%, between 30% and 85%, between 30% and 90%, between40% and 80%, between 40% and 95%, between 40% and 75%, between 55% and65%, between 35% and 90%, between 45% and 80%, between 75% and 95%, orany range within one of the foregoing ranges).

In some embodiments, the methods described herein are used to fabricatefirst element 102 such that the thickness of the element, measured in adirection orthogonal to the planar surfaces of first element 102, isbetween 100-200 μm (e.g., between 110 and 200 μm, between 115 and 200μm, between 120 and 200 μm, between 130 and 200 μm, between 150 and 200μm, between 100 and 190 μm, between 100 and 180 μm, between 100 and 170μm, between 100 and 160 μm, between 100 and 150 μm, between 100 and 140μm, or any range within one of the foregoing ranges).

At step (132) a polymer-based membrane is formed by exposing the surfaceof the first element 102 to a fluorosilane composition to form a layer104 that includes one or more fluorosilane compounds on the surface ofthe first element 102.

A variety of different methods can be used to form layer 104. In someembodiments, for example, layer 104 can be formed in a single stepdip-coating approach (e.g., by dipping first element 102 in a bath thatincludes the fluorinated silane compound to form layer 104 on firstelement 102). Alternative methods for forming layer 104 include, but arenot limited to, spin coating, drop casting, sol-gel methods, andcombinations of any of the foregoing methods. As an example, aplasma-treated first element 102 was dip-coated in a solution thatincluded the fluorinated silane compound3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-Heneicosafluorododecyltrichlorosilane(F21-CH₂CH₂—SiCl₃). The fluorinated silane compound was dissolved in an-hexane solution prior to dip coating. Newly formed membranes can beremoved from the solution that contains the silane compound(s) and dried(e.g., at room temperature for 2 hours).

In some embodiments, a molar percentage of all fluorinated silanecompounds in the membrane that is formed according to the methodsdescribed herein is between 1% and 10% (e.g., between 1.5% and 10%,between 3% and 10%, between 4.5% and 10%, between 5.5% and 10%, between1% and 9.5%, between 1% and 8.5%, between 1% and 8%, between 1% and7.5%, between 1% and 7%, between 1% and 6.5%, between 1% and 6%, or anyrange within one of the foregoing ranges).

It should be noted that although in the foregoing discussion membranesare prepared by first generating active sites on the surface of firstelement 102, this step is optional. Membranes can also be preparedwithout generating active sites, and instead directly forming layer 104on first element 102 using any of the methods described herein.

In some embodiments, the casting solution used to form first element 102can include nanoparticles. For example, as shown in step 126 a of FIG.2B, nanoparticles can be added to the casting solution prior to casting.In general, any of the different types of nanoparticles described abovein connection with FIG. 1 can be added to the casting solution, and thenanoparticles can be added in sufficient concentrations to achieve anyof the different compositions of nanoparticles described above.

As one example, in step 126 a of FIG. 2B, a solution of 5% TiO₂,ultrasonicated in 5 milliliters (mL) of acetone (e.g., C₃H₆O) toachieved homogeneity, was added to the PVDF-based casting solution. Thecasting solution including the nanoparticles was magnetically stirredfor 24 hours at room temperature.

To compare relative performance attributes of the membranes fabricatedaccording to the methods described herein, the wetting behavior ofplasma etched polymer-based membranes 100 after chemical modificationwith different concentrations of a fluorinated silane compound wasmeasured. Active sites were introduced into the PVDF-based first layer102 of each membrane, followed by single step dip-coating of the firstelements 102 to form the membranes

Each membrane was prepared by dip coating a first layer 102 in ahexane-based coating solution, each solution having a differentconcentration of a fluorinated silane compound (e.g., F21-CH₂CH₂—SiCl₃).Solutions with four different amounts of the fluorinated silane compound(e.g., 50 mg, 100 mg, 200 mg, and 400 mg) in n-hexane were prepared andused to fabricate corresponding membranes.

The surface wettability of a PVDF membrane (i.e. no layer 104), achemically modified PVDF membrane (i.e., a membrane formed withoutgenerating active sites before applying layer 104 to first element 102),and a chemically modified PVDF membrane with plasma etched surfaces(i.e., plasma etching of first element 102 followed by dip coating todeposit layer 104) was evaluated by utilizing a drop shape analyzer(e.g., KRUSS, Germany) for water and oil (n-hexane) in an ambientenvironment. A 5 microliter (μL) liquid droplet at ambient temperaturewas placed carefully on the surface of each tested sample and the imagewas recorded. Contact angle measurements were recorded using the sessiledrop method over different regions of the membranes and the averagevalues for each test sample were calculated.

The optimum amount or concentration of the fluorinated silane compoundin layer 104 that achieves the maximum contact angle for water and oilwas determined.

The measured results are shown in FIG. 3 . Increasing the fluorinatedsilane amount in the coating solution from 50 mg to 200 mg resulted inan increase in the static contact angle values for water and oil.Membranes that were both super-hydrophobic and oleo-phobic surfaces wereproduced with approximately 200 mg of the fluorinated silane compoundF21-CH₂CH₂—SiCl₃ in the coating solution.

FIG. 4 is a chart 172 showing the wetting behavior of plasma etchedpolymer-based membranes 100 fabricated with different dip-coating times.The effect of dip-coating times on the contact angles for water and oilfor each membrane were determined. Membranes were fabricated asdescribed above, each with a different dip-coating time (2 h, 4 h, 8 h,16 h, 24 h, and 48 h). The dip coating solution used to fabricate themembranes contained 200 mg of the fluorinated silane compoundF21-CH₂CH₂—SiCl₃, as described above with respect to FIG. 3 .

Increasing the dip-coating time during fabrication resulted in anincrease in the static contact angle values for water and oil, up to adip-coating time of 24 hours, after which a significant increase incontact angle values was not observed, as shown in FIG. 4 . Under theconditions enumerated, the optimal time for the successful formation ofa super-hydrophobic and oleo-phobic PVDF-based membrane was 24 hours.

FIG. 5 is a chart 192 showing the wetting behavior of variouspolymer-based membranes. The results of contact angle measurements areshown for the PVDF membrane without layer 104, the non-activatedchemically modified PVDF-based membrane, and plasma-etched PVDF-basedmembrane discussed above. Water contact angles for the PVDF-basedmembranes with layers 104 were 150.70° and 150.02°, and oil contactangles were 101.36° and 100.1°, respectively, illustrating bothhydrophobic and super-oleophilic characteristics.

In some embodiments, the membranes fabricated according to the methodsdescribed herein can have a water contact angle of between150.01-150.7°(e.g., between 150.03-150.7°, between 150.05-150.7°,between 150.10-150.7°, between 150.2-150.7°, between 150.5-150.7°,between 150.02-150.65°, between 150.02-150.55°, between 150.02-150.45°,between 150.02-150.35°, between 150.02-150.25°, between 150.02-150.15°,or any range within one of the foregoing ranges).

In some embodiments, the membranes fabricated according to the methodsdescribed herein can have an oil (n-hexane) contact angle of between100.1-101.36° (e.g., between 100.15-101.36°, between 100.20-101.36°,between 100.25-101.36°, between 100.3-101.36°, between 100.4-101.36°,between 101-101.36°, between 100.1 and 101°, between 100.1-100.5°,between 100.1-100, or any range within one of the foregoing ranges).

FIG. 6 is a chart 212 showing the wetting behavior of polymer-basedmembranes with TiO₂ nanoparticles and different concentrations of thefluorinated silane compound F21-CH₂CH₂—SiCl₃. The membranes wereevaluated to determine the optimum concentration of F21-CH₂CH₂—SiCl₃ forproducing super-hydrophobic and oleo-phobic characteristics. Asdiscussed above in connection with FIG. 3 , improved hydrophobicity andoleophobicity was observed for membranes produced by dip coating incoating solutions where the amount of F21-CH₂CH₂—SiCl₃ increased up to200 mg. Beyond this amount of F21-CH₂CH₂—SiCl₃ in the coating solution,no significant enhancement in the contact angle values was observed.

FIG. 7 is a chart 232 showing the comparative wetting behavior ofvarious polymer-based membranes with TiO₂ nanoparticles. The surfacewetting behavior of PVDF membranes with TiO₂ nanoparticles, non-etchedPVDF-based membranes with TiO₂ nanoparticles and a layer containing thefluorinated silane compound F21-CH₂CH₂—SiCl₃, and PVDF-based membraneswith TiO₂ nanoparticles, plasma-etched and featuring a layer containingthe fluorinated silane compound F21-CH₂CH₂—SiCl₃, was determined.Contact angle measurement results showed that the surface of PVDFmembranes with TiO₂ nanoparticles was hydrophobic with an average watercontact angle of 100°, but oil wetted the membrane surface. However, thesurfaces of membranes (both etched and non-etched) with TiO₂nanoparticles and the layer containing the fluorinated silane compoundF21-CH₂CH₂—SiCl₃ were super-hydrophobic with a water contact angle of˜150° and oleo-phobic with an oil (n-hexane) contact angle of ˜110°.

Application: The fabricated membranes can be fabricated to control thepermeation water flux, rate of salt rejection, and antifoulingproperties of the membranes. As such, they can be used in variousmembrane-based technologies and applications such as reverse osmosis(RO), forward osmosis (FO), nanofiltration (NF), ultrafiltration (UF),and microfiltration (MF). The membranes are durable and they also haveeconomic advantages due to the simple fabrication method, and thedurability and availability of the constituent materials. The simplefabrication methods described herein can nonetheless yield membraneswith super-hydrophobic and oleo-phobic characteristics. Suitable usesand applications for the membranes include, but are not limited to,different configurations of membrane distillation technology, thetreatment of produced water from petroleum reservoirs, treatment of oilywastewater, treatment of highly saline water with high foulingcharacteristics, treatment of contaminated groundwater, treatment ofbrackish water, and for other applications in which water- and/oroil-repellent surfaces are advantageous or required.

The fabricated membranes can also serve as a barrier between the feedand the permeation sides of a membrane desalination module. They canalso be used for removing volatile organic compounds from water,concentration of acids, bases, and other analytes in a variety ofchemical and biochemical assays, crystallization of dissolved species,separation of azeotropic mixtures of compounds, and for a variety ofselective filtering processes such as dye filtration/removal in textilefabrication processes.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of what may beclaimed, but rather as descriptions of features that may be specific toparticular implementations. Features that are described in thisspecification in the context of separate implementations can also beimplemented, in combination, in a single implementation. Conversely,various features that are described in the context of a singleimplementation can also be implemented in multiple implementations,separately, or in any suitable sub-combination. Moreover, althoughpreviously described features may be described as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can, in some cases, be excised from thecombination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

Particular implementations of the subject matter have been described.Other implementations, alterations, and permutations of the describedimplementations are within the scope of the following claims as will beapparent to those skilled in the art. While operations are depicted inthe drawings or claims in a particular order, this should not beunderstood as requiring that such operations be performed in theparticular order shown or in sequential order, or that all illustratedoperations be performed (some operations may be considered optional), toachieve desirable results.

What is claimed is:
 1. A method for fabricating a polymer-basedmembrane, the method comprising: forming a casting solution comprising apolyvinylidene fluoride (PVDF)-based solvent, and a polyvinylpyrrolidone(PVP)-based modifying agent; dispersing the casting solution to form afirst element; generating a plurality of active sites on a surface ofthe first element; and forming a polymer-based membrane by exposing thesurface of the first element to a fluorosilane composition to form afluorosilane layer on the surface, wherein the fluorosilane compositioncomprises3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heneicosafluorododecyltrichlorosilane(CF₃(CF₂)₉CH₂CH₂SiCl₃).
 2. The method of claim 1, wherein exposing thesurface of the first element to the fluorosilane composition comprisesdip-coating the surface in the fluorosilane composition to form thefluorosilane layer.
 3. The method of claim 1, wherein generating theplurality of active sites comprises plasma etching the surface of thefirst element.
 4. The method of claim 1, wherein forming the castingsolution comprises dissolving a polyvinylidene fluoride-based agent in asolvent.
 5. The method of claim 4, wherein the polyvinylidenefluoride-based agent comprises unsubstituted polyvinylidene fluoride. 6.The method of claim 4, wherein the polyvinylidene fluoride-based agentcomprises at least one substituted polyvinylidene fluoride compound. 7.The method of claim 6, wherein the polyvinylidene fluoride-based agentcomprises unsubstituted polyvinylidene fluoride and at least onesubstituted polyvinylidene fluoride compound.
 8. The method of claim 4,wherein a molar percentage concentration of the polyvinylidenefluoride-based agent in the casting solution is between 5% and 20%. 9.The method of claim 1, further comprising forming the first element bycoagulating the dispersed casting solution in a coagulation bath. 10.The method of claim 1, wherein dispersing the casting solution comprisesdispersing the casting solution on a surface using a casting knife. 11.The method of claim 1, wherein a weight percentage of the fluorosilanecomposition in the membrane is between 5 weight % and 50 weight %.
 12. Apolymer-based membrane comprising: a first element comprising apolyvinylpyrrolidone (PVP) modifying agent and a polyvinylidene fluoride(PVDF)-based compound; and a fluorosilane layer disposed on the firstelement and comprising at least one fluorinated chlorosilane compound,wherein the at least one fluorinated chlorosilane compound comprises3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heneicosafluorododecyltrichlorosilane(CF₃(CF₂)₉CH₂CH₂SiCl₃); wherein a water contact angle for the membraneis greater than 150°; and wherein an oil contact angle for the membraneis greater than 100°.
 13. The membrane of claim 12, wherein a weightpercentage of the at least one fluorinated chlorosilane compound in themembrane is between 5 weight % and 50 weight %.